1. Field of the Invention
The present invention relates to a method and apparatus for indicating a curved line for graphic arts, designation of an extent of an image data on a display panel of a computer device.
2. Description of the Prior Art
As one of methods of indicating a curved line on a display panel of a computer device provided with a coordinate designation device or producing image data indicative of the curved line, there has been proposed a method of connecting a plurality of Bezier curves for indication of a desired curved line. As shown in FIG. 14, the Bezier curves each are in the form of a cubic curve with control points defined by a starting point, a second handle at the starting point, a terminal point and a first handle point at the terminal point in sequence. In the cubic curve, an inclination of a straight line connecting the starting point to the second handle point is defined as an inclination of the cubic curve at the starting point, and an inclination of a straight line connecting the terminal point to the first handle point is defined as an inclination of the cubic curve at the terminal point.
The position coordinate of each control point of the cubic Bezier curves is determined as follows: A position coordinate of a base point P.sub.n-1 defined as a terminal point of a Bezier curve of the number "n-1" and a starting point of a Bezier curve of the number "n" is first set at a position coordinate designated by a coordinate designation device, a position coordinate of a first handle point P.sub.n-1' at the base point P.sub.n-1 is set at a position coordinate designated by the coordinate designation device in such a manner that the inclination of the Bezier curve of the number "n-1" at the terminal point becomes a desired inclination, and the position coordinate of the second handle point P.sub.n-1" at the base point P.sub.n-1 is set at a symmetrical position of the first handle point P.sub.n-1' relative to the base point P.sub.n-1. Subsequently, a position coordinate of a base point P.sub.n defined as a terminal point of a Bezier curve of the number "n" and a starting point of a Bezier curve of the number "n+1" is designated by the coordinated designation device, and a position coordinate of a first handle point P.sub.n' at the base point P.sub.n is designated by the coordinate designation device in such a manner that the inclination of the Bezier curve of the number "n" at the terminal point P.sub.n becomes a desired inclination. With such setting of the position coordinates described above, a plurality of cubic Bezier curves are defined in sequence thereby to indicate a desired curve and to produce data indicative of the desired curve.
When it is desired to change the cubic Bezier curves or to change the data of the cubic Bezier curves, the position coordinate of the second or first handle point for defining each inclination of the cubic Bezier curves at the starting and terminal points is changed to a position designated by the coordinate designation device. In such a conventional method of indicating a desired curve, when the position coordinate of the first handle point P.sub.n-1' at the terminal point P.sub.n-1 is designated by the coordinate designation device, the position coordinate of the second handle point P.sub.n-1 defining the inclination of the following Bezier curve at the starting point is automatically set. When the terminal point P.sub.n of the following Bezier curve and first handle point at the terminal point P.sub.n are designated, the inclination of the following Bezier curve at the starting point is retained in a condition defined by the second handle point P.sub.n-1. Accordingly, such a Bezier curve as shown by a dot and dash line in FIG. 14 may not be defined. As a result, renewal of the position coordinate of the second handle point at the terminal point has been frequently required for defining a desired Bezier curve. In addition, variation of a Bezier curve caused by change of a position coordinate of each handle point may not be directly estimated by a user. It is, therefore, difficult for the user to change the display of the Bezier curve to a desired curve.